1. Field of the Invention
This invention relates to a plastic lens substrate and also to an apparatus for and a method of the production of such lens substrate.
2. Related Background Art
Heretofore, a plastic block material suited as a plastic lens substrate for use in eyeglasses has been produced from processing of one of its two surfaces by means of three processing steps, i.e., grinding, smoothing and polishing. The plastic block material (referred to hereinafter as the block material) denotes a thick-walled lens having already been processed on a convex side but being still required to be processed so as to gain a lens thickness and a concave side as desired. The block material is a so-called semi-finished lens. Details as regards each of the process steps stated above will be described below.
Firstly, the grinding step is intended to cut one selected surface of the block material (hereinafter called a lens forming surface) in such a manner that a desired radius is attained on the lens forming surface. To this end, a diamond wheel is employed which is known as a cup-shaped tool having diamond particles electro-deposited on its grinding face. To conduct the grinding step, the diamond wheel is caused to axially rotate and then brought into movable contact on the particulate diamond-deposited face with the lens forming surface of the block material, while a coolant is being applied to the lens forming surface. The coolant is usually water or an aqueous solution in which a rust preventive, an antifoaming agent and the like are contained.
The smoothing step follows upon completion of the grinding step.
The smoothing step is intended to render fine or smooth the lens forming surface of the block material, which lens forming surface has been cut to a predetermined radius but with some surface roughness through the grinding step. In the conventional practice, the smoothing step is effected by use of an aluminum tray provided with a face having a radius to correspond to that defined on the lens forming surface of the block material. Disposed adhesively over that tray face is a sheet-like polishing medium commonly called a smoothing pad for exclusive use in such a step of smoothing. The aluminum tray mentioned here is known as a processing tray. The processing tray is manipulated to rotate such that during application of a coolant, the polishing medium is allowed to slidably contact with the lens forming surface of the block material.
The block material processed to have reduced surface roughness on the lens forming surface through the smoothing step is thereafter subjected to the polishing step as a finishing operation.
The polishing step contemplates imparting greater fineness or smoothness to the lens forming surface having undergone processing via the smoothing step. Also in the polishing step, use is made of an aluminum tray of the type stated previously in connection with the smoothing step having a face radially shaped to fit the lens forming surface of the block material. A polishing medium (a polishing pad, for example) to be adhesively mounted on the aluminum tray is softer in nature and smaller in mesh than that employed in the smoothing step. The polishing step is achieved by bringing the resulting processing tray into slidable contact with the lens forming surface, while a polishing liquid is being applied to the latter.
In the production of a plastic lens substrate, the block material has been processed on its lens forming surface by means of the three process steps as discussed above. In each such step, each individual exclusive processing apparatus is employed.
To produce a plastic lens substrate for use in glasses, the present inventors have practiced the production apparatus and production method of the prior art, thereby processing a plastic block material. Consequently, these inventors have found that the conventional practice suffers from too long a period of time for processing.
The conventional production method needs a separate processing apparatus so as to carry out each process step of grinding, smoothing and polishing of a plastic block material. Such method, therefore, leads to an increase in equipment cost and in floor space. Additionally, many different processing trays are needed to cope with varying radii desired to be processed in both of the smoothing and polishing steps. Those processing trays cause burdens of time and cost for their preparation. Furthermore, the radii to be processed are dependent upon the formulations of lenses that are variable with the requests to be made by individual users. A large number of radii are required to be satisfied in order to tailor lenses to different users needs. However, because of a great expenditure of time and effort to prepare processing trays, inventories are necessary for those trays of a wide variety. This results in added storage space and increased cost investment in advance. The foregoing problems create an obstacle to improved lens productivity and to saved production cost.
Besides, and disadvantageously, the conventional production method using processing trays requires that the trays be brought into slidable contact with a lens forming surface of a plastic block material with the result that possible processing is limited to a spherical lens and a toric lens.